Adenosine acts as a neuromodulator in the circulatory, endocrine, immune, and central nervous system. The A.sub.1 -adenosine receptor subtype is generally inhibitory toward adenylate cyclase, and the A.sub.2 - is stimulatory. Other effector systems in particular ion channels may be regulated by A.sub.1 and A.sub.2 receptors. Potent and selective analogs of adenosine receptor agonists (adenosines) and antagonists (xanthines) have been developed using the "functionalized congener" approach (J. Med. Chem. 1985, Vol. 28, p. 1334 and p. 1341). Analogs of adenosine receptor ligands bearing functionalized chains were synthesized and attached covalently to various organic moieties, such as amines and peptides. N.sup.6 -[p-(Carboxymethyl)phenyl]adenosine and 8-p-(carboxymethyloxy)phenyl derivatives of 1,3-dialkylxanthines were synthesized as biologically active functionalized agonists and antagonists, respectively. Attachment of polar groups to the xanthine congeners increased water solubility. Amino congeners derived from the carboxylic acid derivatives of agonists or antagonists had nanomolar A.sub.1 -receptor binding affinities, as measured by inhibition of binding of tritiated N.sup.6 -cyclohexyladenosine on rat cerebral cortex membranes. Conjugates of the various functionalized congeners may be useful as high affinity receptor probes in histochemical, chromatographic, or radioligand binding studies.